Biological interconversions between the various forms of simple forms of simple nitrogen compounds are fundamentally significant processes within the global nitrogen cycle. Metalloproteins within numerous microorganisms play a key role in effecting these transformations and are thus important in controlling the balance of simple nitrogen-containing molecules on earth. Imbalances in the relative concentrations of such compounds in the environment can have significant effects on ecosystems and human health, thus providing impetus for the study of how the controlling metalloproteins operate. Copper-containing enzymes are important in denitrification, a central process in the biological nitrogen cycle whereby several classes of microorganisms utilize oxidized nitrogen compounds as terminal electron acceptors in respiration. Specifically, copper centers in enzymes are involved in several steps in the conversion of nitrate (NO3-) and nitrite (NO2-) to gaseous products (NO, N20 and/or N2). Although there have been advances in understanding aspects of the active site structures of such enzymes, the plausibility of various postulated pathways for the complicated chemistry that they perform has not been determined. In addition, a general lack of knowledge of the fundamental chemistry of copper-NxOy species has hindered attempts to better under-stand how nitrogen oxides bind to and am activated by biological copper centers. The proposed research is designed to address this deficiency by focusing on the synthesis, fun physical and spectroscopic characterization, and detailed study of the reactivity of copper-nitrogen oxide complexes that model the active sites of copper-containing enzymes important in the biological nitrogen cycle. Particular emphasis will be placed on the construction of mononuclear Cu(I) and Cu(II) complexes of NO, N02-, N203 -, and N202-2- supported by multidentate, biomimetic, and sterically hindered ligands. Delineation of the active site features of analogous protein active sites will be accomplished through in-depth studies of the structures and reaction chemistry of these synthetic model compounds.